This drill is called "spotting". This is one of the most important skills to perfect during a pirouette. It gives the pirouette an asthetic sharpness, helps the dancer balance themself so they can keep track of where their body is in space. The turn of the head must not be delayed ! There has to be a strong accent of the head, an attack. If the head stays too long, before whipping it around, the neck muscles begin to pull the head back slightly. By attacking the spot, the dancer can more easily keep her vertebrae in line, to prevent from becoming dizzy.
1) Stance: Keep your head up and in line with your neck. Eyes should not be focused down, keep them level. If you are a beginner, you might want to practice in the mirror, but keep in mind dancers perform on stage where there aren't any mirrors, so keeping your eye on an object or something level with your vision, is the best idea.
2) Preparation: Prepare yourself to turn your body and whip your head around. It is important to keep your shoulders squared off and your head aligned.
3) Follow through: Your body will be turning around a Longitudinal Axis(vertical axis) in the Transverse Plane. Snap your head around as quickly as possible , prepare yourself to look at that same spot your eyes were focused on before you started your pirouette.
4) Landing: Finish exactly where you had started. Eyes level, you should not feel dizzy or off balance.
Keep practicing this drill. With improvement, you can perform this drill twice, three, four times in a row, in order to practice more than a single pirouette.